Capsules for pharmaceutical preparations and neutraceutical preparations are generally prepared using gelatin, hydroxypropyl methylcellulose (HPMC) or hydroxypropyl methylcellulose phthalate (HPMCP) as base materials.
Gelatin capsules have high industrial productivity and high price competitiveness. However, if gelatin capsules contain 10 wt % moisture or less, they may lose plasticity and may show serious deterioration in impact resistance. Concern on mad cow disease has limited the use of gelatin capsules. For these reasons, plant-based HPMC capsules prepared without gelatin, and enteric HPMCP capsules have drawn attention.
However, conventional HPMCP capsules have low transparency and are fragile at a low moisture content of 10 wt % or less, for example. In addition, when being stored in severe high-temperature conditions, the HPMCP capsules may lose interior moisture slowly and harden because they include an excess of a neutralizing agent (e.g., alkaline agent). Furthermore, the neutralizing agent may separate from the hardened capsules, which is also called a separation of salt, within a relatively short period of time, mostly within 30 days.